1. Field of the Invention
The present invention relates to diffractive optical elements, and in particular, to a diffractive optical element suitable for use with light having plural wavelengths or bandwidths and to an optical system including such diffractive optical element.
2. Description of the Related Art
In order to reduce chromatic aberration, in addition to a method of utilizing a combination of lenses of different glass materials, a method using a diffractive optical element disposed on a surface of a lens or arranged as part of an optical system is known. This diffractive method is disclosed in, for example, SPIE, vol. 1354, International Lens Design Conference, 1990; Japanese Patent Laid-Open No. 4-213421 (corresponding to U.S. Pat. No. 5,044,706); and Japanese Patent Laid-Open No. 6-324262 (corresponding to U.S. Pat. No. 5,790,321).
This method utilizes a physical phenomenon in which chromatic aberration of a light ray having a reference wavelength at a refractive face is present in a direction opposite to that at a diffractive face in an optical system.
The use of such a diffractive optical element can achieve an effect equal to that realized by an aspherical lens by changing a period of the periodic structure of the diffractive optical element, resulting in a large reduction in aberration.
In refraction, a single ray of light is still a single ray after being refracted. In contrast to this, in diffraction, a single ray is divided into a plurality of diffraction orders after being diffracted. Therefore, in the case of using a diffractive optical element as a lens system, it is necessary to determine the grating structure so that a pencil of light in a used wavelength range is concentrated in a single specific order (hereinafter, referred to also as “design order”). If light rays concentrate in a specific order, the intensity of a diffraction ray of a different order is low. If the intensity is reduced to zero, such a diffraction ray is not present.
Therefore, in order to apply a diffractive optical element to an optical device using broadband light, such as a camera for photographs, the diffraction efficiency of light of a design order must be sufficiently high throughout the used wavelength range.
In the case where a light ray of a diffraction order different from a design order is present, the light ray is imaged at a location different from that for a light ray of the design order and thus results in flare. Accordingly, in an optical system utilizing a diffraction effect, it is important to fully consider the spectral distribution of diffraction efficiency at a design order and the behavior of a light ray of an order different from the design order.
FIG. 14 shows a diffraction grating of a known diffractive optical element. FIG. 15 illustrates the characteristics of diffraction efficiency for a specific diffraction order when the diffractive optical element shown in FIG. 14 is disposed on a certain surface.
In the following description, each value of diffraction efficiency represents the ratio of each diffraction ray to the overall transmitted pencil of light in light quantity, and for the sake of simplicity, the value ignores reflected rays at a grating surface.
In FIG. 15, the horizontal axis of a graph represents wavelength and the vertical axis represents diffraction efficiency. This diffractive optical element is designed such that it exhibits the highest diffraction efficiency in a used wavelength range at the positive first diffraction order, as indicated by a solid line. In other words, the design order is the positive first order.
In FIG. 15, the diffraction efficiency for two diffraction orders around the design order (i.e., zeroth order and positive second order, which are one order lower than and higher than the positive first order, respectively) is also shown.
As shown in FIG. 15, the diffraction efficiency for the design order exhibits the highest value at a certain wavelength (hereinafter, referred to as “design wavelength”) and decreases gradually at other wavelengths.
The decrease in diffraction efficiency for the design order leads to diffraction rays of other orders and results in flare.
There exists a known structure capable of reducing the decrease in diffraction efficiency (see Japanese Patent Laid-Open No. 9-127322, corresponding to U.S. Pat. No. 6,157,488). As shown in FIG. 16, in this structure, the two materials of a first diffraction grating 6 and a second diffraction grating 7 and the grating thicknesses, d1 and d2, thereof are optimally selected, and the diffraction gratings 6 and 7 having the same grating pitch are arranged adjacent to each other with an air layer 8 disposed therebetween. Therefore, this structure achieves high diffraction efficiency throughout the visible range, as shown in FIG. 17.
This structure is of a two-layer construction, as shown in FIG. 16, and realizes high diffraction efficiency by optimizing the refractive index and the dispersion of each of the materials of the diffraction gratings 6 and 7 and the grating thicknesses d1 and d2.
Japanese Patent Laid-Open No. 9-127322, which describes the known structure described above, merely discloses that a plane produced by connecting the tips of the grating part is flat and does not mention a specific shape of a diffraction grating disposed on a curved surface.
There are known structures of a diffractive optical element in which a plurality of diffraction gratings disposed on curved surfaces are layered, the diffractive optical element realizing high diffraction efficiency (see Japanese Patent Laid-Open No. 2001-042112 corresponding to European Patent Application Publication No. 1 072 906 A2, and Japanese Patent Laid-Open No. 2002-107520 corresponding to U.S. Patent Application Publication No. 2002-036827).
FIG. 19 is a cross-sectional view showing a main portion of a diffractive optical element disclosed in Japanese Patent Laid-Open No. 2001-042112.
As shown in FIG. 19, in a diffractive optical element 1, a first diffraction part 2 and a second diffraction part 3 are arranged adjacent to each other with an air layer 8 disposed therebetween. The first diffraction part 2 includes a first substrate 4 and a first diffraction grating 6 disposed on a surface of the first substrate 4. The second diffraction part 3 includes a second substrate 5 and a second diffraction grating 7 disposed on a surface of the second substrate 5. All layers function as the single diffractive optical element 1.
The surfaces, on which the diffraction gratings 6 and 7 are disposed, of the substrates 4 and 5 and the opposite surfaces thereof are both curved surfaces. Each of the substrates 4 and 5 itself functions as a refractive lens. Envelope faces 9 and 10 are curved surfaces defined by connecting the tips of the first diffraction grating 6 and that of the second diffraction grating 7, respectively.
FIG. 20 is a cross-sectional view showing a main portion of a diffractive optical element disclosed in Japanese Patent Laid-Open No. 2002-107520. As shown in FIG. 20, in a diffractive optical element 1, a first diffraction part 2 and a second diffraction part 3 are arranged adjacent to each other with an air layer 8 disposed therebetween. The first diffraction part 2 includes a first substrate 4 and a first diffraction grating 6 disposed on a surface of the first substrate 4. The second diffraction part 3 includes a second substrate 5, a second diffraction grating 7 disposed on a surface of the second substrate 5, and a third diffraction grating 16 disposed on the second diffraction grating 7. All layers function as the single diffractive optical element 1.
A face 10, which is opposite to a grating surface 12, of the third diffraction grating 16, is a curved face that does not include a grating part, and has substantially the same curvature as a face on which a grating part is formed in the second substrate 5. For the substrates 4 and 5, the surfaces on which the diffraction gratings 6 and 7 are disposed and the opposite surfaces are both curved surfaces. Each of the substrates 4 and 5 itself functions as a refractive lens.
These layered diffractive optical elements, in which a plurality of diffraction gratings are arranged on curved faces, are optimized for a certain angle of incidence. Specifically, in the example shown in FIG. 19, the diffractive optical element has a structure optimally adjusted for a case where a pencil of light is incident mainly from the direction normal to the envelope faces 9 and 10 (hereinafter, referred to also as “curved base faces”), which are individual faces defined by connecting the tips of a grating part 6a and a grating part 7a. 
Diffractive optical elements applied to various optical systems receive rays incident from various directions, i.e., may receive a pencil of light incident from directions other than the direction normal to an envelope face (curved base face) of a diffraction grating. For example, with a diffractive optical element including a diffraction grating having an envelope face with a small radius of curvature, a pencil of light is incident from various directions.
In this case, an angle from the direction normal to the envelope face to one direction becomes significantly larger with respect to the optical axis. Therefore, in the case where the diffractive optical element is used in an optical system, incident angles are distributed from the direction normal to the envelope face to a direction parallel to the optical axis.
As a result, the structure in which the diffractive optical element is optimized for a pencil of light incident from the direction normal to the envelope face may have low diffraction efficiency.